Method and process for prediction of subsurface fluid and rock pressures in the earth

ABSTRACT

A method of determination of fluid pressures in a subsurface region of the earth uses seismic velocities and calibrations relating the seismic velocities to the effective stress on the subsurface sediments. The seismic velocities may be keyed to defined seismic horizons and may be obtained from many methods, including velocity spectra, post-stack inversion, pre-stack inversion, VSP or tomography. Overburden stresses may be obtained from density logs, relations between density and velocity, or front inversion of potential fields data. The seismic data may be P-P, P-S, or S-S data. The calibrations may be predetermined or may be derived from well information including well logs and well pressure measurements. The calibrations may also include the effect of unloading. The determined pressures may be used in the analysis of fluid flow in reservoirs, basin and prospect modeling and in fault integrity analysis.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.09/433,446 filed on Nov. 4, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to geophysical exploration andmore particularly to methods for accurately estimating fluid andoverburden pressures in the earth's subsurface on local, prospect andbasin-wide scales.

2. Background of the Art

During drilling of a borehole, drilling fluids, usually referred to as“mud,” are circulated in the borehole to cool and lubricate the drillbit, flush cuttings from the bottom of the hole, carry cuttings to thesurface, and balance formation pressures encountered by the borehole. Itis desirable to keep rotary drilling mud weights as light as possible,but above the formation pore fluid pressure, to most economicallypenetrate the earth; heavier muds may break down rocks penetrated by theborehole and thereby cause loss of mud. Mud weight is carefullymonitored and may be increased during drilling operations to compensatefor expected increases in the formation fluid pressure. In some areas,however, there may be unexpected abnormal increases in pressure, withdepth such that mud weight does not compensate pressure; the result canbe blowout of the well.

Normal pressures refer to formation pressures that are approximatelyequal to the hydrostatic head of a column of water of equal depth. Ifthe formation were opened to the atmosphere, a column of water from theground surface to the subsurface formation depth could balance theformation pressure. In many sedimentary basins, shallow predominantlysandy formations contain fluids that are under hydrostatic pressure.

At a number of offshore locations, abnormally high pore pressures havebeen found even at relatively shallow sub-sea bottom depths (less thanabout 1500 meters). This could occur if a sand body containing largeamounts of water is covered by silt or clay and buried. The dewateringof clays may result in the formation of relatively impermeable shalelayers that slow down the expulsion of water from the underlying sand.The result of this is that the sand may retain high amounts of fluid andthe pore pressure in the sand exceeds that which would normally beexpected from hydrostatic considerations alone, i.e., the fluid pressureexceeds that which would be expected for a column of water of equivalentheight. This phenomenon of overpressuring is well known to those versedin the art and is commonly referred to as “geopressure.”

It is desirable to set casing in a borehole immediately above the top ofa geopressured formation and then to increase mud weight for pressurecontrol during further drilling. Setting a casing string which spansnormal or low pressure formations permits the use of very heavy drillingmuds without risking breaking down of borehole walls and subsequent lostmud in the shallower interval. On the other hand, should substitution ofheavy drilling mud be delayed until the drill bit has penetrated apermeable overpressurized formation (e.g., sandstone), loss of wellcontrol or blowout may occur.

In areas where there is reason to suspect existence of such highpressure formations, various techniques have been followed in attemptsto locate such geopressure zones. For example, acoustic or electric logshave been run repeatedly after short intervals of borehole have beendrilled or are acquired using measurement-while-drilling techniques, anda plot of acoustic velocity or electrical resistance or conductivity asa function of depth has been made. Abnormal variations of acousticvelocity and/or electrical properties obtained by logging may indicatethat the borehole has penetrated a zone of increasing formationpressure. Such techniques are very expensive and time-consuming andcannot predict what pressures will be encountered ahead of the bit.

Several methods are known in the art for estimating pore pressures informations, using well log data and also from seismic surveyinformation. One such method is well known in the art as the “Eaton”method, and is described at Eaton, “The Equation for GeopressurePrediction from Well Logs” SPE 5544 (Society of Petroleum Engineers ofAIME, 1975). The Eaton method of determining pore pressures begins withdetermination of the so-called “normal compaction trend line” based uponsonic, resistivity, conductivity, or d-exponent data obtained by way ofwell logs. The normal compaction trend line corresponds to the increasein formation density (indicated by sonic travel time, resistivity orconductivity) that would be expected as a function of increasing depthdue to the increasing hydrostatic pressure that forces fluids out fromthe formations and thus decreases the sonic travel time (increases thevelocity), assuming the absence of geopressure. This normal compactiontrend line may be determined solely from the sonic travel time.conductivity, or resistivity well log data, or may be adjusted toreflect extrinsic knowledge about the particular formations of interest.The Eaton method calculates pore pressure by correlating the measureddensity information, expressed as an overburden gradient over depth, todeviations in measured sonic travel time, (or electrical resistivity orconductivity) from the normal compaction trend line at specific depths.The pore pressure calculated from the Eaton equations has beendetermined to be quite accurate, and is widely used in conventional wellplanning.

Specifically, the Eaton method determines a pressure gradient accordingto the relation $\begin{matrix}{G_{p} = {G_{o} - {\left( {G_{o} - G_{n}} \right)\left\lbrack \frac{\Delta \quad t_{normal}}{\Delta \quad t_{observed}} \right\rbrack}^{3}}} & (1)\end{matrix}$

where G_(P) is the formation pressure gradient (psi/ft), G_(O) is theoverburden gradient, G_(N) is the normal gradient, Δt_(normal) is thenormal transit time and Δt_(observed) is the observed transit time.

However, application of the Eaton method has been limited to theimmediate locations of existing wells, as it depends on well log data.It is of course desirable to estimate pore pressure at locations at thesites of proposed new wells, and thus away from currently existingwells, particularly to identify locations at which production will beacceptable at a low drilling cost (e.g., minimal use of intermediatecasing). In addition, knowledge of pore pressure at locations away fromexisting wells enables intelligent deviated or offset drilling, forexample to avoid overpressurized zones.

Kan (U.S. Pat. No. 5,130,949) teaches a method in which seismic data iscombined with well log data to generate a two-dimensional geopressureprediction display; this permits deviated and horizontal well planningplus lithology detection. Shale fraction analysis, compaction trend, andseismic velocity may be automatically or interactively generated on acomputer work station with graphics displays to avoid anomalous results.Corrections to velocity predictions by check shots or VSP, andtranslation of trend curves for laterally offset areas increasesaccuracy of the geopressure predictions. In particular, Kan '949determines the transit time from sonic logs for compressional waves inpredominantly shaly sections and expresses the pore pressure gradient(PPG) in terms of the transit time departure from the compaction trendline, δΔt, as

PPG=0.465+C ₁(δΔt)+C ₂(δΔt)²  (2)

Coefficient C₁ typically varies from 0.002 to 0.02 if the transit timeis expressed in microseconds per foot and the PPG is measured in psi/ft.C₂ may be positive or negative.

Kan '949 also teaches the use of vertical seismic profile (VSP) data forcalibration of the sonic log data. Kan (U.S. Pat. No. 5,343,440) andWeakley (U.S. Pat. No. 5,128,866) further teach the use of coherencyanalysis of surface seismic data for determination of intervalvelocities.

Scott (U.S. Pat. No. 5,081,612) teaches a variation of the Eaton methodin which an equation of the form

V _(c) =V ₁(1−a ₁ L−a ₂ φ+a ₃ P)  (3)

where a₁, a₂ and a₃ are constants, V₁ is a constant, V_(c) is calculatedvelocity, L is the lithology of the formation, φ is the porosity and Pis the effective pressure (difference between the overburden pressureand the formation fluid pressure). The compaction of the sediments isgoverned by an equation of the form

φ=φ₀ e ^(−a) ₄ ^(P)  (4)

A reference model for the sedimentary basin is developed assumingcompaction under hydrostatic pore pressure. A reference effectivepressure and a reference velocity profile are obtained. An iterativeprocedure is used in which the lithology may be varied with depth andthe reference velocity profile is compared to a velocity profileobtained from seismic data.

In addition to undercompaction, Bowers (U.S. Pat. No. 5,200,929)discusses a second cause of overpressuring. Abnormally high pressure canalso be generated by thermal expansion of the pore fluid (“aquathermalpressuring”), hydrocarbon maturation, charging from other zones, andexpulsion/expansion of intergranular water during clay diagenesis. Withthese mechanisms, overpressure results from the rock matrix constrainingexpansion of the pore fluid. Unlike undercompaction, fluid expansion cancause the pore fluid pressure to increase at a faster rate than theoverburden stress. When this occurs, the effective stress decreases asburial continues. The formation is said to be “unloading.” Since sonicvelocity is a function of the effective stress, the velocity alsodecreases and a “velocity reversal zone” develops. A velocity reversalzone is an interval on a graph depicting sonic velocity as a function ofdepth along a well in which the velocities are all less than the valueat some shallower depth.

A large portion of the porosity loss that occurs during compaction ispermanent; it remains “locked in” even when the effective stress isreduced by fluid expansion. A formation that has experienced a greatereffective stress than its current value will be more compacted and havea higher velocity than a formation that has not. Consequently, therelationship between sonic velocity and effective stress is no longerunique when unloading occurs. In other words, for every effectivestress, there is no longer one unique sonic velocity. The sonic velocityfollows a different, faster velocity-effective stress relationshipduring unloading than it did when the effective stress was building.This lack of uniqueness is called “hysteresis.” Since fluid expansioncauses unloading, while undercompaction does not, hysteresis effectsmake the sonic velocity less responsive to overpressure generated byfluid expansion. As a result, the pore fluid pressure corresponding to agiven sonic velocity at given depth within a velocity reversal zone canbe significantly greater if the overpressure was caused by fluidexpansion rather than undercompaction. Therefore, the sonic velocity ofan overpressured formation is indirectly dependent upon both themagnitude and the cause of overpressure. To account for different causesof overpressuring, Bowers teaches the use of two differentvelocity-effective stress relations: one relation applies when thecurrent effective stress is the highest ever experienced by asubterranean formation and a second relation that accounts forhysteresis effects is used when the effective stress has been reduced.Pore fluid pressure is found by subtracting the computed effectivestress from the overburden stress. Bowers uses a relationship of theform

V=C+A[σ _(max)(σ/σ_(max))^((1/U))]^((1/B))  (5)

for the effect of unloading. In eq. (5) σ_(max) is the maximum stress towhich the rock has been subjected. The unloading curve parameter U is ameasure of how plastic the sediment is, with U=1 and U=∞ defining thetwo limiting cases. For U=1, the unloading curve is the same as theloading curve whereas for U=∞ the velocity remains fixed at a valueV_(max) determined by the stress σ_(max).

SUMMARY OF THE INVENTION

The invention provides a methodology, process and computer software forthe prediction of fluid and rock pressures in the subsurface usinggeophysical and geological data. The method includes techniques forvelocity analysis from seismic data that are used to drive the pressureprediction, as well as an integrated approach to deriving pressure datathat is new and novel in nature and scope. The invention addresses theprediction of pressure information for three scales of analysisincluding (1) basin-scale (10-500 km spatial lengths) analysis ofhydrocarbon systems, (2) prospect-scale (1-10 km spatial length)analysis of fluid flow that can be used to analyze fluid movement inlocalized areas, and (3) prediction of pressure conditions at a specificlocation (0-1 km spatial length) where a well is to be drilled. Theresults of the prediction can be utilized in a range of otherapplications that address the fundamental behavior of hydrocarbonsystems and can improve the ability to find commercial quantities ofhydrocarbons in the subsurface. The results can also be used to designand optimize well planning.

The technique and computer software estimates fluid pressure (fromoverburden stress and seismic velocities), fracture gradient (wheneffective stress=0), overburden pressure (from integration of densitylogs, using the Gardner-Gardner-Gregory relations with velocityestimates from sonic logs or seismically derived seismic velocities, ora combination of seismic data and Gravity/Magnetic data), effectivestress (from seismic data) and porosity (from seismic data) for wellplanning, basin-scale pressure fields and prospect-scale pressurefields, and also can generate predictions and interpretations that areapplicable to:

hydrocarbon maturation

fluid migration

lateral and vertical seal rock integrity

reservoir-specific lateral pressure changes

fault-seal properties

effects of hydrocarbon reservoirs on pressure prediction

effects of anomalous lithologic intervals on pressure prediction andoverburden estimation

The software provides an operating environment in which all datapertinent to predicting subsurface rock and fluid pressures can bestored, displayed in tabular and graphical forms, analyzed, calibratedand used in the prediction process.

The software provides the user with three pressure predictionalgorithms, the Eaton method, the effective stress method, and theequivalent depth method, and allows a range of curve fitting options foreach method including linear, power law, exponential, and quadraticfunctions.

The computer software allows large amounts of seismic velocity data tobe processed and analyzed with the results being displayed on a displayof a migrated or stacked seismic section. The software allows seismicdata from an entire 2-D seismic line, a set of multiple 2-D seismiclines, or a 3-D seismic volume to be processed and displayed with therelated seismic and well data.

The computer software allows calibration functions and predictioncomputations to be constrained by geologic boundaries such as horizonsand faults so that multiple calibrations can be applied to areas withcomplex geological loading histories.

The technique and computer software use multiple data types and handlethem in an integrated fashion. The data include seismic and well-basedvelocity information (such as from VSP data and sonic logs), geologicboundaries such as horizons and faults mapped from the seismic data,pressure data from wellbores, well log data, and digital seismic data instandard industry formats.

The computer software has the ability to read data in generic textformats and in standard formats such as would be generated by welllogging contractors, seismic processing contractors, or commercialseismic interpretation systems. The data can be displayed in varioustabular and graphical forms. The software provides display capabilitythat includes interactive viewing and analysis of multiple data typessimultaneously so that the user can evaluate all of these data at once.Related data can be calibrated using a variety of functional equationsthat are appropriate for pressure prediction.

The technique and software contains a calibration module that includes amethod and means to display, edit, and datum a variety of well logs, andfit calibration curves to the logs or estimate equation coefficients andexponents for a specific prediction method (such as the Eaton method,the Bowers method, the effective depth method). The displays includedepth versus log value displays along with cross-plot displays forvarious log parameters and plots of velocity versus effective stress.The depth and crossplot displays are linked interactively to ascrollable coefficient display window that allows the user to modify thecoefficients of the equation and observe the resulting change in thegraphical display in real time. The calibrations can be performed usingall of the methods and equations known to experts versed in the artincluding but not limited to the Eaton method, the effective stressmethod and the equivalent depth method. The calibration module includesa method and means for using a variety of mathematical fits includinglinear, exponential, power law and quadratic forms that encompass thefull range of possible mathematical forms for pressure relationships inthe subsurface. The calibration module allows the calibration toequation form of effective stress versus velocity, velocity versusdensity, velocity versus porosity, density versus porosity, and fracturegradient versus depth. These equations can be defined for any subset ofa set of well log data that the user selects, for example, on the basisof a lithological or other discriminator applied to the data, or byselecting a specific geological interval that is defined by certaincharacteristics and specific mapped geologic boundaries.

The calibration module also allows the calibration of unloadinghysteresis for zones where the velocity has decreased due to secondarypressure conditions. This is determined by fitting the velocity andstress data for the unloaded zone to a relationship for unloading.

The calibration module includes interactive graphical displays ofvelocity, pore pressure, effective stress, overburden and fracturegradient versus depth and or time with horizons and faults posted on thedisplays.

The technique and software contains a prediction module that includes amethod and means to display seismic velocity data along with displays ofstacked or migrated seismic data with horizons, faults and an attributeof choice from a range of velocity and pressure data types plus the samecalibration displays above.

The prediction module applies the calibration curves or equations to thedata at each seismic velocity location in order to calculate suchattributes as pore pressure, effective stress, fracture gradient,overburden, and porosity.

The prediction module can store in memory the calculated attributes asfunctions of time or depth for later display and analysis.

The prediction module displays the stacked seismic data with interpretedhorizons and faults, and pore pressure or other attribute underpaymentin color during the analysis that can be used interactively during theprediction of pressures.

The prediction module includes interactive graphical displays ofvelocity, pore pressure, effective stress, overburden, density, porosityand fracture gradient versus depth and or time with horizons and faultsposted on the displays, seismic sections and base maps.

The technique and software contains a method and means to generatedigital files of the predicted attributes in appropriate formats forother mapping packages, and transfer these files to a computer mediumappropriate for transfer to other computer software. Prediction resultscan be output in time or depth for subsequent import into seismicinterpretation systems.

The software contains a method and means to display seismic scaleoverlay plots of pressure and other attributes and depth plots for eachlocation analyzed. The predicted attributes include, but are not limitedto the following:

pore pressure as a function of depth or time

effective pressure as a function of depth or time

overburden as a function of depth or time

fracture gradient as a function of depth or time

porosity as a function of depth or time

excess pressure as a function of depth or time

unloading pressure as a function of depth or time

The technique and software includes a method and means to use velocitiesfrom one or more sources including but not limited to one or more of thefollowing:

stacking velocity data

coherency inversion velocity data

pre-stack inversion P-wave velocity

post-stack inversion P-wave velocity

pre-stack inversion S-wave velocity

post-stack inversion S-wave velocity

shear-wave stacking velocity data

tomographic P-wave velocities

tomographic S-wave velocities

VSP velocity data

VSP look-ahead inversion

sonic logs

dipole sonic logs

mode-converted shear wave velocities

combined Vp and Vs inversion

The method includes use of all of the above velocity types for pressureprediction with horizon-keyed constraints. The technique also claimsfirst use of some of the velocity types above without horizon-keyedconstraints. These include the following:

pre-stack inversion P-wave velocity

post-stack inversion P-wave velocity

pre-stack inversion S-wave velocity

post-stack inversion S-wave velocity

shear-wave stacking velocity data

tomographic P-wave velocities

tomographic S-wave velocities

VSP look-ahead inversion

dipole sonic logs

mode-converted shear wave velocities

combined Vp and Vs inversion

The technique and software includes a method and means for pickingseismic stacking velocities using a horizon-keyed and fault-keyedapproach. This means that, unlike conventional seismic-velocity pickingmethods that use the strongest semblances from the seismic data, thevelocities are picked to honor the formation geological and structuralboundaries defined by the lithological rock units and such features asfaults and anomalous body boundaries in 2-D or 3-D.

The technique and computer software includes a method to isolate thevelocities for geologic intervals that contain hydrocarbons orlithologies other than those that are appropriate to use in theprediction step. This same method can be used to isolate the velocitiesof reservoirs and seal rocks that are not in equilibrium with theencasing seal rocks. This method requires velocity types that havesufficient resolution to accurately detect the distinct velocities inthese zones. In this situation, the software allows the user to identifythe zone or zones to be excluded, and the software removes this valuefrom the calculations in the prediction step.

The software also allows the data from anomalous zones to be saved to aseparate data file that can be analyzed separately for pressures inspecific zones.

The method can also be applied to pressure prediction from the resultsof seismic analysis on the geologic hazard called shallow water flows asdefined in U.S. patent application Ser. No. 09/433,446 filed on Nov. 4,1999 having the same assignee as the present application and thecontents of which are fully incorporated herein by reference.

The method of excluding zones from the pressure analysis can be appliedfor any velocity data types that may be developed and used in othersoftware that are already known to those versed in the art of pressureprediction.

The technique and software includes a method and means to change theoverburden and fracture gradient calculated for a given location basedon one of three schemes. In the first scheme, the analyst identifies ananomalous density zone that may be caused by the presence of eitheranomalous rocks or fluids, and inputs to the software the anomalousdensity value for that zone. The software inserts this new density forthe interval into the overburden function for the location beingevaluated, and re-calculates the overburden function using this zone inplace of the densities from the reference function that was usedoriginally to determine the overburden stress versus depth. This newfunction with the anomalous zone is used only at the location where theanomalous formation was identified. By doing this at each location wherethe anomalous zone is observed, the user may build an overburden andfracture gradient profile that honors the actual geology in 3 dimensionsrather than relying on a 1D density function determined from a singlewell location that does not always represent the subsurface at otherlocations.

The second scheme for changing the overburden and fracture gradient isto use 2D or 3D density models determined independently from theinversion of vector and tensor gravity and vector and tensor magneticdata and to integrate these models in 2D or 3D to predict overburden inan area where pressures are to be predicted. This can include, but isnot limited to the inclusion of 1D well density data for calibration ofthe gravity and magnetic data. Such methods of determination ofoverburden are discussed, for example, in U.S. patent application Ser.Nos. 09/285,570, 09/399,218, and 09/580,863.

The third scheme for changing the overburden and fracture gradient is touse density values predicted in 2D or 3D seismic data using simultaneousinversion of multicomponent PP and PS data or the density valuespredicted from pre-stack inversion. In this case, the seismic data iscalibrated with density data from well control, and the 2D or 3Dseismically derived density values are used at each location tocalculate overburden and fracture gradient.

The technique and software contains a method and means for creating andapplying calibration functions that are controlled by and keyed tospecific geologic intervals in the subsurface. These are usually appliedto a geologic interval of specific age that is observed to occur acrossa specific area of the subsurface and has a distinct pore pressurecalibration that is different from intervals above and below it in thesubsurface. The method and means allows the user to identify interpretedhorizons to which the calibration is keyed and limits the application ofthe calibration to those horizons.

The method and means allows the user to use more than one activecalibration at one time and the computer software splices these multiplecalibrations together in the displays so that the user cansimultaneously produce an integrated prediction for multiple zones withdifferent calibrations. This multiple-zone calibration and predictionalso includes the application of unloading parameters that can bespecified for each calibration zone.

The technique and software includes interactive help menus that guidethe interpreter through the process and method for performing pressureprediction. The help menu is divided into a methods tutorial and ahands-on help manual that defines and specifies what each module andfunction in the software actually does.

The technique and software includes a method and means of designing andapplying a transformation function to data sets that are referenced todifferent map coordinate systems, including but not limited to UTM, shotpoint and line number, inline number and cross-line number, and stateplane systems, such that all data can be referenced to a commoncoordinate system. Precise design and application of the transformationsare facilitated by real-time parameter iteration made possible byconcurrent examination of a base map display together with access to thetransformation menu.

The technique and software includes a method and means of organizingdata in a hierarchical structure. At each level of the hierarchy, oneand only one set of data can be active. Within the active data set,multiple data elements can be selected for graphical and computationalpurposes.

The technique and software includes a method and means to automaticallypropagate the effects of interactive editing of calibration functions tothose graphical displays that depend on the edited function. Thiscapability includes but is not limited to real time updating ofcalculated effective stress, normal trend velocity, and pore pressurevalues in the calibration module in response to edits to the velocityversus effective stress calibration function, and updating the colorunderlay displays in the seismic section display in response to edits toparameters in the prediction module.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features that are believed to be characteristic of theinvention, both as to organization and methods of operation, togetherwith the objects and advantages thereof, will be better understood fromthe following detailed description and the drawings wherein theinvention is illustrated by way of example for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention:

FIG. 1 is an illustration of a vertical section through the earthshowing a well and geologic horizons including faults.

FIG. 2 is an example of a flow-stream using some of the modules of thepresent invention.

FIG. 3 is a diagram illustrating the identification of shales at a welllocation.

FIG. 4a is an example of a- cross-plot of density and porosity

FIG. 4b illustrates two different examples of calibration curvesrelating the density and porosity data of FIG. 4a.

FIG. 5 shows an example of overburden stress determination that isuseful in calibration.

FIG. 6 shows examples of plots of velocity and porosity as a function ofdepth along with calibration curves.

FIG. 7 shows plots of velocity, effective stress and pore pressuregradient.

FIG. 8a is a plot showing a comparison of seismically derived intervalvelocities and log velocities.

FIG. 8b is a plot showing a comparison of seismically derived intervalvelocities and filtered log velocities.

FIG. 9 is a display showing seismic data and locations where seismicvelocities have been derived.

FIG. 10 is a display of seismically derived interval velocities at twodifferent locations along with a filtered velocity log from a well.

FIG. 11 shows a comparison of densities derived from seismic velocitiesand a density log.

FIG. 12 shows sonic log velocities, effective stresses in the formationand pore pressure data from mud weights.

FIG. 13 shows an example of a calibration of sonic velocity andeffective stress using a Bowers curve fit.

FIG. 14 shows effective stresses derived from seismic velocities usingthe calibration of FIG. 13.

FIG. 15 shows a comparison of an Eaton calibration with a Bowerscalibration.

FIG. 16 shows the use of velocity spectra in obtaining seismicvelocities over defined seismic interval and a comparison with prior artvelocity spectra.

FIG. 17 is a schematic illustration of the effects of unloading.

FIG. 18 is an illustration of the equivalent depth method.

FIGS. 19a-19 c illustrate overpressuring that may occur in a thin sandbody as a result of rapid burial.

FIGS. 20a-20 b is an example of seismic data over a geologically complexprospect and pressure predictions made using the present invention.

FIG. 21 similar to FIG. 2, is another example of a flow-stream usingsome of the modules of the present invention

FIG. 22 similar to FIGS. 2 and 21, is another example of a flow-streamusing some of the modules of the present invention.

FIG. 23 similar to FIGS. 2, 21 and 22, is yet another example of aflow-stream using some of the modules of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an example of a vertical section of subsurfaceregion 1 is shown Indicated in the figure is a well location 25 with awell 27 penetrating the subsurface. A number of faults 10, 11, 12, 13,14, 15, 16, 17, 18 are indicated in the figure as well as a number ofhorizons 21 that correspond to geologic intervals of interest.

In the well 27, a plurality of measurements may be made of theproperties of the subsurface formations penetrated by the well. Thesetypically include sonic logs that measure the velocity of compressionaland shear velocities, density logs, gamma ray logs that are indicativeof the shale content of the formation, and resistivity logs of varioustypes that measure the formation resistivity.

In addition to these logs, a record is kept of the mud weight that isused for the drilling of the wellbore: as noted above, the mud weight isusually selected to maintain a slightly overbalanced condition whereinthe borehole fluid pressure is slightly in excess of the formation fluidpressure. After the well is drilled, pressure drawdown and pressurebuildup tests may be run at selected depths to make an accuratedetermination of the formation fluid pressure. These are indicative ofthe fluid pressures encountered in the well.

A line of 2-D seismic data (not shown) may be acquired over the region.Alternatively, a plurality of 2-D seismic lines or a set of 3-D seismicdata may be acquired over the surface of the earth that includes thesection shown in FIG. 1.

In one embodiment of the present invention, using the log informationand/or seismic information in the vicinity of the well, a determinationof the effective pressure in the formations and the overburden pressureon the formations is made. In another embodiment of the invention, theseismic data are used to obtain estimates of the effective stresses inthe subsurface at locations away from the well. This may be done in theimmediate vicinity of the well; on a prospect scale location (e.g.between the faults 11 and 12); or on a basin wide scale (e.g.,extrapolating to another prospect such as the region between the faults13 and 18.

In the present invention, the interpreter has a plurality of modulesthat perform different functions to enable this process of extrapolationaway from the well. Depending upon the nature of the data available,different combinations of these modules may be used by the interpreter.Rather than providing an exhaustive list of the various combinationsthat may be used by the interpreter, the description herein firstprovides examples of the processes performed by the various modules andthen gives examples of sequences that may be used.

Turning now to FIG. 2, an overview of one method of using the presentinvention is illustrated. The input data 51 includes logs, pressuremeasurements at the well, velocity functions characterizing thepropagation of seismic waves in the subsurface, seismic data, seismichorizons picked on the seismic data and surface elevations (for landseismic data) and water depth (for marine seismic data). These aredescribed in more detail below.

The logs that may be used in the present invention include conventionalsonic logs that measure the compressional velocity of elastic waves inthe formation. These would be known to those versed in the art and arenot described further. In addition to conventional sonic logs, anembodiment of the present invention also uses measurements of shearvelocities in the formation. These may be direct measurements of shearvelocities using devices such as a dipole logging tool. U.S. Pat. No.4,606,014 to Winbow et al. gives an example of a dipole sonic loggingtool. Other examples of dipole logging tools are described in U.S. Pat.No. 4,703,460 to Kukjian et al., U.S. Pat. No. 4,862,991 to Hoyle et al.and are not discussed further. Alternatively, direct measurements ofshear velocities may also be obtained using a quadrupole or octupolelogging device. Such devices are described in U.S. Pat. No. 4,855,963 toWinbow et al. and U.S. Pat. No. 4,951,267 to Chang et al. and are notdiscussed further.

Shear velocity measurements of the formation may also be obtained byindirect methods. For example, U.S. Pat. No. 4,774,693 to Winbow et alteaches a method for determination of shear velocities using guidedwaves produced in a borehole by a logging tool. In the presentinvention, shear velocities obtained by any direct or indirect methodmay be used.

Shear velocities are an important indicator of the effective stress of afluid filled rock. U.S. patent application Ser. No. 09/433,446 ofHuffman having the same assignee as the present application and thecontents of which are fully incorporated herein by reference teaches therelation between effective stress and the shear velocity of clasticsediments. An optional embodiment of the present invention uses thisrelationship to obtain an estimate of effective stress from shearvelocity measurements.

Density logs may also be used in the present invention. They may be usedto obtain an estimate of the compressional velocity using theGardner-Gardner-Gregory GGG) relation

ρ=0.23 V ^(0.25)  (6)

where ρ is the density in gm/cm³ and V is the compressional wavevelocity in ft/s. An optional embodiment of the present invention uses ageneral power law relationship in which the exponent and the constant ofproportionality may be determined from a combination of density logs andsonic logs. The density logs may also be used in the determination ofthe overburden stress as described below.

Logs indicative of the shale content of the formations (such as gammaray logs) may also be part of the input to the present invention. Asdescribed below, portions of the sonic logs corresponding to shalyintervals may be used as part of the calibration process. In a preferredembodiment of the invention, the shale-prone intervals are selectedpreferentially and are used to define a shale compaction trend in thecalibration process.

Mud weight measurements may also be recorded and used along with RepeatFormation Tester (RFT) and Modular Dynamic Tester (MDT) for estimates ofthe formation fluid pressure. The MDT is the new Schlumberger tool thatdoes the same thing as the RFT tool, but also has several new featuresincluding (1) a sensor in the device that tests fluid resistivity tomake sure that the formation fluid being gathered is pristine formationfluid, (2) a larger set of sample chambers for gathering fluid, and (3)a pump that actually draws fluid out of the formation rather thanrelying on ambient flow to fill the chambers. (Probably more than youwanted to know)

The use of velocity information from seismic data has been described inprior art. However, the use of such velocity information in prior art islimited to conventional coherency spectra of gathers of common-midpoint(CMP) data. In the present invention, a novel aspect is the use of suchconventional coherency spectra in conjunction with picked horizons ofthe seismic data. This is illustrated below and discussed with referenceto FIG. 16. In addition, alternative embodiments of the presentinvention use other methods for determination of velocities of seismicwaves in the subsurface. Specifically, in land seismic prospecting,shear seismic data may be used and velocity spectra derived therefrom.In addition, in marine seismic prospecting, a combination ofcompressional and converted wave seismic data may be used fordetermination of compressional and shear velocities. The use ofconverted wave prospecting for velocity determination is discussed, forexample, in U.S. Pat. No. 4,881,209 to Bloomquist et al., U.S. Pat. No.6,128,580 to Thomsen and is not discussed further here.

Alternative embodiments of the present invention use velocities derivedfrom prestack inversion of compressional wave data, post-stack inversionof compressional wave data, prestack and post-stack inversion of shearseismic data, joint inversion of compressional and converted waveseismic data and joint inversion of compressional and shear seismicdata. An example of prestack inversion of compressional wave seismicdata is given in U.S. Pat. No. 5,583,825 to Carrazonne et al. thecontents of which are incorporated herein by reference. Other examplesof velocity determination using inversion techniques are given in Savicet al., Mallick et al., Connolly, and Duffaut et al.

Post-stack inversion is one alternative to conventional velocityanalysis that provides higher resolution by inverting for impedance fromthe reflection strength. Post-stack inversion allows the analyst toseparate the seismic wavelet from the reflection series represented bythe geologic formations, and results in an estimate of residualimpedance for each layer. Post-stack inversion can be applied using onlythe stacked seismic data, or can be calibrated with well logs, checkshot surveys, VSP data and seismic velocity data. When calibratedproperly with low frequency velocity field data, the analysis can beused to generate an estimate of the absolute impedance, or itscomponents of velocity and density. This requires a good set ofdensity-velocity relationships for the lithologies encountered in thewells, so that these two components can be separated effectively. Thisexercise is not trivial, however, because the post-stack inversiontechnique ignores the fact that offset-dependent behavior (AVO) isburied in the stacked response and can cause significant perturbation ofthe results. One way to overcome this limitation and also boostresolution of the results at the same time is to use only the nearoffset traces for the analysis. This is a good method to use as itprovides higher resolution results due to the removal of far-offset datathat are degraded by NMO stretch. Near-stack inversion also gives themost robust calibration to well logs that are essentially measuring thesame vertical-incident information.

In an optional embodiment of the present invention, pre-stack inversionof seismic data provides a higher level of accuracy and allowsprediction of pressure at a scale that was not achievable in the past.Pre-stack inversion can estimate the P wave velocity, shear wavevelocity, and density simultaneously by using the near-offsetreflectivity and amplitude versus offset behavior of each reflection inthe subsurface. This allows the interpreter to estimate the overburdenand effective stress from the same data set. The resolution limits forpre-stack inversion are approximately at the tuning thickness of theindividual formations, so that pressure data can be generated for layerson the order of 100-200 feet thick at moderate depths in clastic basins.The biggest drawback to pre-stack inversion at this time other than costis its extreme sensitivity to data quality. A robust inversion requiresdata that are relatively clean, uncontaminated by multiples and noise,and in areas of little structural complexity. In addition, theresolution and accuracy of pre-stack inversion is only as good as thequality of the reflection events in the data. If there are pressurecells that do not have reflections associated with them, no velocity orreflection-amplitude technique including inversion will identify thosezones.

The use of calibrated pre-stack inversion, especially wheremulticomponent data are available, allows the estimation of density andvelocity simultaneously along a line. This makes possible predictionsthat take into account the lateral variations in density that accompanychanges in pressure. At present, most methods of pressure predictionassume that the overburden from the control well represents overburdenglobally, which is false in general. Pre-stack inversion helps to remedythis problem by predicting the density from the seismic data andallowing the pressure interpreter to make judgements about the densityusing the seismic and well data and his own intuition.

Pre-stack inversion makes possible isolation of velocities forindividual sand packages so that the interpreter can determine wheredisequilibrium may exist between the sand-bearing formations and massiveshales, and to isolate the velocity and density effect ofhydrocarbon-bearing reservoirs on the velocity field around them. Atpresent, most methods lump these effects into thicker stratigraphicintervals that have a single velocity attached to them and hide theeffect. These errors can cause predictions to overestimate orunderestimate pressures significantly which leads to less effective wellplanning.

Velocity information for the present invention may also be obtainedusing cross-well tomography. This is generally has poorer resolution ina horizontal direction but may have higher resolution in a verticaldirection than analysis of velocity spectra of surface seismic data.Depending upon the type of sources and detectors used, compressionaland/or shear velocities may be obtained. Such tomographic methods wouldbe known to those versed in the art and are not discussed further.

Velocity information in a limited region in the vicinity of a wellboremay also be obtained using Vertical Seismic Profiling (VSP). These maybe for P-wave velocities or for S-wave velocities and may be either in aconventional VSP or in a look-ahead mode wherein velocities are obtainedpertaining to formations ahead of a drilled well. Such methods would beknown to those versed in the art and are not discussed further.

Returning now to FIG. 2, one of the steps that may be used incalibration is to identify the shale formations in the well 53. Thereason for this is that clay and shale tend to show the effects ofcompaction better than sand. FIG. 3 shows an example of theidentification of the shales in the subsurface. The left panel 101 showsa sonic log 111, the center panel 103 shows an example of a porosity log113 and the right panel 105 shows an example of a gamma ray log 115. Theporosity log 113 is obtained external to the present invention from thedensity log. The density porosity is preferable to a neutron porosity.The gamma ray log 115 is a good indicator of shaliness in clasticsediments because shales have a higher potassium content than sands as aresult of which shales have a higher ganmna ray count. Indicated in theright panel 105 is a shale line 117 that may be determined interactivelyby the user of the invention. In a preferred embodiment of theinvention, the shale line may be represented by a piecewise linearfunction. Only velocity and porosity samples that correspond to depthswhere the gamma ray count is to the right of the shale line 117 are usedin the calibration process. An optional plot in the present invention(not shown) provides the interpreter with a color display of theselected and deselected portions of the various logs.

A useful aspect of the present invention is the ability to cross-plotdifferent properties. An example of this is shown in FIG. 4a that is across-plot of density along the abscissa 131 and porosity as theordinate 133. The present invention provides the user with the abilityto interactively edit the data by drawing a polygon such as 135 enablingthe selection of points within the polygon and deselecting pointsoutside the polygon. As with other displays, the selected and deselectedpoints appear in different colors.

FIG. 4b shows the data of FIG. 4a with two different curves 137 and 139superposed thereon. In the example of FIG. 4b, the curve 137 correspondsto the GGG relation whereas the curve 139 is a different fit determinedinteractively by the interpreter. The interpreter has a variety of fitsavailable, including power laws, exponentials and polynomials. One ofthe useful aspects of the present invention is that in the interactivedisplays, an active curve or data points appear in one color (e.g.,green) and the results of user interaction appear in a second color(e.g., red). This enables the interpreter to easily compare the currentiteration with what may have been determined earlier. Different curvesmay be derived in different regions of the plot.

Cross-plots and calibrations such as shown in FIGS. 4a-4 b are alsoindicated at 59 in FIG. 2: different calibrations may be derived by theinterpreter for the density-velocity relation and the porosity-velocityrelation.

Still referring to FIG. 2, the present invention provides theinterpreter with the ability to determine the overburden stress 55. Thisis illustrated in FIG. 5. Two panels of data are shown. The right panel151 is a plot of density (abscissa) vs. the depth 161 (ordinate). Thedensity values from the density log are shown 162: these may or may nothave been preselected as described above. Also shown is an example of apower law curve 163 that starts with a density of 1.4 at the oceanbottom. Additional points 164 may be added by the interpreter.

Shown in the right panel 153 is the determined overburden stress. Twocurves are shown: 167 corresponds to the curve density-depth valuesshown in the left panel 151 while the curve 165 corresponds to a uniformdensity of 2.0 above the top of the log. As in other displays, theinterpreter has the capability of changing the parameters of the curvefit and immediately seeing the effect this has on the determinedoverburden stress.

Another display that is important is shown in FIG. 6. The right panel203 is a cross-plot of density against velocity. This is the same datathat is in FIG. 4b; however, two types of symbols are used in the plot.The light colored symbols are for the shale samples selected in FIG. 3while the dark symbols are used to plot the remaining values from thelogs of FIG. 3. The left panel 201 is a cross-plot of velocity againstporosity, with all the data points shown. Also shown in the right panel203 are two different curve fits to the data: the curve 205 correspondsto a density constrained to be 1.4 at the ocean bottom (velocity=5000ft/s) while the curve 207 is an unconstrained fit.

Turning now to FIG. 7, another display that is useful in performing thecalibration is shown. The left panel 225 is a plot of the velocity log231. In the actual screen display, the shales and the non-shales wouldappear in different colors. The right panel 229 is a plot of theoverburden stress 233 on a pore-pressure gradient (PPG) scale. Alsodisplayed in the right panel are mud weight data from the scout data andthe mud log 235, 237. The center panel 227 on a scale of stress showsthe overburden stress 239 and the effective stress 241. The effectivestress is based on an assumption that the fluid pressure is hydrostatic.The difference in the appearance of the overburden stress in the center227 and the right panel 229 is because 227 is in pressure and 229 is inpressure gradient units. The reason the effective stress appears as afat line is that it comprises diamond shaped plot symbols every 50 ft.

Those versed in the art would recognize that sonic log velocities do notnecessarily agree with seismically determined velocities in theimmediate vicinity of the well. This is brought out in FIGS. 8a and 8 b.FIG. 8a is a plot of the seismically derived interval velocity 255 andthe sonic log velocity 257. The abscissa 251 is the velocity and theordinate 253 is depth. FIG. 8b is similar to FIG. 8a except that thesonic log displayed is a filtered log 257′. They clearly show that theseismic velocities are higher than the sonic logs.

The reason for this discrepancy becomes clear upon examination of FIG. 9which is a display of the seismic section with the well location 271indicated, along with the location 273 where the velocity used in thedisplay of FIGS. 8a, 8 b was derived. The well is seen to be on a flankof a salt dome 279 with steeply dipping sediments on its flanks. Inaddition to the dips, the shallow seismic data at location 273 is noisy,possibly due to gas leakage from a reservoir. FIG. 10 shows a display ofthe filtered sonic log 257′ along with the seismic velocity 255 atlocation 273 and the seismic velocity 285 from a location 275 in FIG. 9away from the salt dome. The velocity function 285 is in good agreementwith the filtered sonic log 257′.

Another useful display in the calibration process is a comparison ofdensities derived from seismic velocities with the density log at thewell. This is shown in FIG. 11 where the left panel 301 is identical toFIG. 10 and the right panel shows the density log 305 at the well, anddensity functions 307 and 309 derived from the seismic velocities atlocations 273 and 271 in FIG. 9 respectively. The density function isderived from a density-velocity calibration such as curves 205 or 207 inFIG. 6. A similar plot may be made (not shown) of seismically derivedporosities and well porosities.

In the present invention, a useful check of the calibration process isto compare seismically derived velocities, densities and porosities withlog data from another well if such a well is available. Such acomparison serves as an indication of how far from a calibration wellthe seismic data may be used for prediction of formation properties. Ifsuch a comparison is good over certain geologic intervals but not overother geologic intervals, this enables the interpreter to use adifferent calibration for different regions of the subsurface region 1in FIG. 1.

FIG. 12 is a plot similar to FIG. 7. The left panel 325 shows the soniclog 331 as a function of depth. The right panel 329 is a display on apressure gradient (mud weight) scale of the overburden pressure gradient333 and the mud weight observations at specific depths 337. The centerpanel is a display on a stress scale of the overburden stress 335 andthe effective stress 337′ corresponding to the observed mud weights 337.Using the data from the middle panel 327 a display of the effectivestress calibration may be obtained as shown in FIG. 13. Shown in FIG. 13is a plot of the sonic velocity as a function of the effective stress.The velocity data points corresponding to the mud weight observations337′ are plotted as the points 337″. The horizontal scale is a stressscale and a fit curve 351 is indicated on the display. As indicated inthe figure, the interpreter has control over the type of curve fit(linear, quadratic, exponential or Bowers) 353 and the parameters of thefit are shown on the screen display.

Using the results of the calibration curve 351 in FIG. 13, seismicallyderived velocities such as those shown in panel 301 of FIG. 11 may beused to estimate the effective stress. This is shown in FIG. 14 wherethe left panel 352 shows the seismically derived velocities 357 near thewell location and the calibration curve for normal pressure, the centerpanel 353 shows the estimated effective stress curve 359 along with adisplay of the overburden stress calibration 361. The right panel 355shows, on a pore pressure gradient scale, the estimated pore pressurecurve 359′ along with the overburden stress gradient 363. As can be seenin FIG. 13, the effective stress is well below what would be expectedunder hydrostatic conditions, indicating that the entire interval isoverpressured. A display similar to FIG. 14 but not shown here may beused to shown a comparison between the effective stress expected on thebasis of the sonic logs with the expected stress undernormally-pressured conditions: such a comparison may be used prior tothe prediction of effective stresses from seismically derived velocitiesshown in FIG. 14.

In a manner similar to that described immediately above for the Bowerscalibration, the present invention also provides a capability for doinga calibration based on the Eaton relationship. This is not describedhere, but the end result of an Eaton calibration is shown in FIG. 15 asa comparison with the Bowers calibration. As in FIG. 14, the left panel375 shows a velocity plot, the middle panel 377 is a plot of effectivestress and the right panel 379 is a pore pressure gradient plot. Themiddle panel shows the effective stress 381 from the Eaton calibrationand the right panel shows the pore pressure gradient 381′ from the Eatoncalibration. It may be seen that the Eaton calibration does not matchthe data points for the mud weight below about 5000 ft. in both thepanels.

The above discussion dealt with the use of data from a wellcorresponding to a specific calibration location within the areal extentof the seismic data with or without seismic velocity data for thespecific well location. As an alternative to deriving the calibrationcurves, the present invention includes the capability of using apreviously determined calibration curve in the analysis of the seismicdata using regional information or information from a location that isoutside the areal extent of the seismic data.

The above discussion also dealt with the use of density data from welllogs to establish an overburden stress. U.S. patent applications Ser.Nos. 09/285,470 filed on Apr. 12, 1999; Ser. No. 09/399,218 filed onSep. 17, 1999; and Ser. No. 09/580,863 filed on May 30, 2000, all havingthe same assignee as the present application and the contents of whichare fully incorporated herein by reference, teach the use of potentialfields data, including scalar, vector or tensor gravity and/or magneticdata, in the determination of densities of subsurface formations inconjunction with seismic data. The teachings of these applications maybe used in the present invention for obtaining densities of subsurfaceformations; as discussed in the aforesaid applications, the densitymodel may be 1-D, 2-D, 2.5-D or 3-D. In addition, U.S. patentapplication Ser. No. 09/405,850 filed on Sep. 24, 1999 and having thesame assignee as the present application, teaches the use of potentialfields data in combination with seismic data for obtaining estimates ofoverburden stresses and effective stresses in subsurface formations. Theteachings of the '850 application may also be used in the presentinvention.

As mentioned above, seismic velocities may be derived in the presentinvention using many methods. The discussion above relating to FIGS.8-15 could use velocities from any of these methods. In one embodimentof the invention, the velocities may be derived without reference toseismic horizons defined by the interpreter on a seismic section. Inanother embodiment of the invention, the seismic velocities are definedwith reference to seismic horizons defined by the interpreter.

One such method that has been used in the past is the use of velocityspectra for the determination of interval velocities of compressionalwaves. However, these have been done in prior art without reference todefined seismic horizons. FIG. 16 shows how velocity spectra may beobtained with referencing to defined seismic horizons. The left panel401 shows conventional velocity spectra for compressional wave data. Thevertical axis is seismic travel time and the horizontal axis is themoveout velocity. Plotted in the panel 401 are coherency values of theseismic data at different moveout velocities and times: such spectrawould be well known to those versed in the art. In prior art methods,peaks of the spectra such as at times indicated by 405 a, 405 b, 405 c,. . . 405 n are picked and seismic interval velocities derived fromthese peaks using known methods. As would be known to those versed inthe art, for causes beyond the scope of the present invention, thesepeaks do not necessarily coincide with geologically meaningful seismichorizons. The right panel 403 shows the same velocity data with seismictimes indicated as 407 a, 407 b, 407 c . . . 407 k that correspond toseismic horizons on a seismic section (not shown). In the presentinvention, coherency values at these times 407 a, 407 b . . . are pickedto define the moveout velocity as a function of depth. These do notnecessarily correspond to local peaks of the velocity spectra. The useof horizon keyed velocity spectra of compressional waves is a novelaspect of the present invention. The other methods of determination ofseismic velocities described above are believed to be novel with orwithout the use of horizon keyed picking.

The present invention also has the capability of accounting for theeffect of unloading of the effective stress in the subsurfaceformations. The effect of unloading is best understood with reference toFIG. 17.

This diagram is a 3-dimensional view that demonstrates the interplaybetween velocity 503, porosity 501 and effective stress 507. The loadingpath starts at an effective stress of zero, and the velocity increasesand porosity decreases until the material changes over from a Wood'sEquation material to a frame-bearing clastic rock that can support aneffective stress on the grains. The Wood's Equation portion of theloading path 511 occurs as the material is initially deposited andcompacted near the surface. Once the critical porosity such as 512 isreached, the material follows the primary compaction curve 513,achieving either a compacted or undercompacted state. If allowed tocompact normally with fluid draining out of the pore spaces, a rock willcontinue up the normal loading path and velocity will increase andporosity will decrease. Both of these properties are dependent on theeffective stress on the grains that are bearing the external load. If atsome point the fluid is prevented from escaping, the rate of ascent upthe normal pressure curve will decrease so that the rock has a lowervelocity and effective stress than would be expected at normal pressureconditions at a given depth of burial. This condition is known asundercompaction or compaction disequilibrium. The key to understandingundercompaction is to recognize that a rock under these conditions stillremains on the normal compaction trend, only it is not as compacted asyou would expect it to be at that depth of burial under normalhydrostatic pressure.

Unlike undercompaction, a rock subjected to secondary pressure (alsocalled unloading) cannot stay on the normal compaction curve. When fluidis pumped into a rock or expands within the pore spaces in the rock, thecompaction process is arrested and the rock begins to display a form ofhysteresis behavior in velocity-effective stress space. When thisoccurs, the porosity essentially doesn't change except for some minorelastic rebound (Moos and Zwart, 1998), and the velocity behavior isstrictly controlled by the contact area and the grain-to-grain contactstresses in the rock. Because there is essentially no porosity change,the net effect is to flatten out the velocity-effective stress trend andproduce an unloading trend that is different from the compaction trend.The unloading curve must start from the velocity-porosity-effectivestress point on the primary compaction curve where the unloading begins.This is why unloading 515 always starts from aporosity-velocity-effective stress point on the primary compactioncurve. Note that the unloading paths occur essentially in thevelocity-effective stress plane as the porosity decrease associated withcompaction is arrested during unloading and very little elastic rebound(less than 1 porosity unit) occurs during the unloading process. As theeffective stress decreases due to higher fluid pressures at fixedoverburden, the velocity decreases in direct relation to the stresschange. Once a rock is on an unloading path, the rock doesn't changeporosity unless other phenomena such as diagenesis or cementation areoccurring concurrently with the pressure changes. For the rock to begincompacting again, the secondary pressures must first bleed off, or theoverburden must increase sufficiently by additional sediment loading tocounterbalance the secondary fluid pressures that were added to therockmass and increase the effective stress. In either case, the rockwill respond to the change in effective stress and will move back up theunloading path 515 until it contacts the normal compaction curve again.Once the effective stress has exceeded the value where unloading began,the rock can begin to compact again. If the stress never reaches thislevel, the rock will remain on the unloading path indefinitely. It isimportant to recognize in this context that the normal compaction curveis also the maximum compaction, maximum velocity and minimum porositythat a material can achieve at normal pressure for a given effectivestress.

To properly predict pressure ahead of the bit, it is essential to knownot only the normal compaction trend, but also the slope of thesecondary pressure curve and the maximum stress-velocity state that wasachieved before unloading began. It is important to recognize that thepresence of two possible pressure mechanisms and a range of possiblemaximum velocities for unloading leads to a range of possible predictedpressures according to which mechanism is assumed to be at work andwhere unloading began. For any velocity, there are a range of possiblepressures that are a function of the normal trend, the maximum velocityattained by the rock, and the unloading curve slope. In practical terms,for any observed velocity value, the minimum pressure case isrepresented by the normal trend curve (equivalent depth of burial) andthe maximum pressure case is represented by the greatest reasonablemaximum velocity on the normal trend and the slope of the unloadingcurve from that point back to the observed velocity value. Thus, it isimperative that the pressure prediction expert be aware of both causesof pressure and also recognize when and how to apply unloading.

Turning now to FIG. 18, the equivalent depth method is illustrated. Thismethod is applicable only for undercompaction and not for modeling theeffect of unloading. Shown is a shale parameter (e.g., effective stress,velocity, etc.) plotted as a function of depth. A number of data points527 a, 527 b . . . 527 i . . . 527 n are shown along with the normalcompaction trend curve 525. Due to rapid burial, the points below 527are undercompacted, as indicated by the reversal of the trend data. Datafrom the depth 531 are have the same properties as data along the normalcompaction curve at a depth 529. The depth 529 may be referred to as theequivalent depth to 531.

An isolated sand layer within a thick shale that is subjected to rapidburial may have very unusual stress configurations. This is illustratedin FIGS. 19a-19 c (after Stump et al). Consider a sand body 551 as shownin FIG. 19a that is initially in a horizontal position and then due torapid burial at the right end, assumes the configuration shown by 551′in FIG. 19b. Consider now the relative pressures between the sand andthe shale at the shallow end (points 555, 553) and the deep end (points556, 554). Normal hydrostatic and lithostatic stress distributions areindicated in FIG. 19c by 571 and 573 respectively. The shale 553 at theshallow end is essentially at hydrostatic pressure given by the point553′ while the shale at the deep end 554 is at an abnormally highpressure denoted by the point 554′. (If the subsidence is rapid enough,the shale follows a stress line 575 parallel to the lithostatic line573). The sand at the deep end will now be at a pressure denoted by 556′but due to the good permeability of the sand, the pressure gradientwithin the sand will be substantially hydrostatic and the shallow end ofthe sand will now be at a pressure denoted by 555′. As a result of this,the stress in the sand is greater than the stress in the adjoining shaleand, if the difference is large enough, this can lead to a breakdown ofany possible sealing strength of the sand-shale interface and anyhydrocarbons that may be present in the sand will leak out. Seismicinversion techniques discussed above (prestack, post-stack, PP, PS) areparticularly useful in identifying trap integrity problems involvingthin sand layers. The use of these methods to isolate the velocity of ahigh-permeability reservoir zone and then predict pressures as afunction of depth or structural position is a preferred embodiment ofthe present invention.

The stresses determined by using the method of the present invention canthus be used as an indicator of trap integrity in hydrocarbonreservoirs.

Referring now to FIGS. 20a and 20 b, the seismic data corresponding toFIG. 1 are shown in FIG. 20a. The position of the well 25 of FIG. 1 isshown at 625 along with the location of a second well 627, and thelocations of the various faults and horizons shown in FIG. 1. FIG. 20bis a display of the interpreted fluid pressure in the region encompassedby the seismic data. Based on the display, fault 610 shows a major faultseal failure near the horizontal position indicated by 650 while thefault 612 shows a major fault seal failure near the horizontal positionindicated by 652: the pressure is not discontinuous across the faults atthe indicated location. In contrast, the figure shows a pressurediscontinuity across the fault 613 indicative of fault seal integrity.There also appears to be a somewhat less serious fault leak across thefault 618.

Those versed in the art would recognize that trap integrity may also beused in the vertical direction: a pressure discontinuity across a knownreservoir caprock is indicative of vertical integrity of a seal. On theother hand, if pressure appears to be continuous across a caprock, thenthe trap integrity is questionable.

Use of such displays on a basin wide scale is clearly useful in basinmodeling. In addition, using geologic information, it may be possible toidentify source rock intervals on a prospect scale or a basin-widescale. Abnormally high fluid pressures in such source rock intervals areindicative of a secondary buildup of pressure caused by source rockmaturation and a subsequent expulsion of hydrocarbons into the rockmatrix.

The pressure displays along with knowledge of lithologies associatedwith different subsurface formations can provide information about themigration of fluids in the subsurface. This is an important diagnosticin prospect evaluation as a trap with a large “drainage” area is likelyto contain more hydrocarbons than one with a small drainage area. Thepressure displays may also be used to estimate lateral pressure changeswithin a reservoir. The pressure displays may also be used in theplanning of drilling of wellbores: abnormally high fluid pressuresindicated on seismic data would indicate the necessity of using highermud weight in the drilling of a well. This may be used in the analysisof wellbore stability. Abnormally high fluid pressures indicated onseismic data near the ocean bottom are a warning of shallow intervalswhere blowouts may occur.

Returning now to FIG. 2, the present invention also has the capabilityof producing maps 59. Any of a plurality of parameters may be displayedon a base map in a manner suitable for helping an interpreter analyze aprospect. For example, displays may be made of a parameter between anytwo seismic horizons, at any depth or at any seismic reflection time. Tofacilitate this display, location transforms may be applied 57 on thewell positions, the positions of seismic lines, seismic horizons,velocity functions etc. Methods for applying these transforms andproducing a suitable map would be known to those versed in the art andare not discussed further.

Given the variety of modules and options in the present invention, bynow those skilled in the art would have recognized that other flowcharts besides the one shown in FIG. 2 may be used. FIG. 21 shows a flowchart of operations that are particularly suitable for processing largeamounts of data with a minimum of human interaction. Data are input 701relating to velocity functions, horizons, elevations (for land seismicdata) and bathymetry (for marine data), and locations of seismic lines.Predetermined calibrations are input 703 describing the overburdenstress as a function of depth and a relation between seismic velocityand effective stress using, e.g, the Bowers, Eaton or other suitablemethod. Optionally, a density-velocity and porosity-velocity relationsmay be input 705. Using the velocity functions from 701 and thepredetermined calibrations, predictions are made of parameters that mayinclude the pore pressure, density and porosity using any of the methodsdescribed above. The predicted parameter values 709 may be exported foruse elsewhere 717, and/or displayed on a seismic section 715. Inaddition, maps may be produced 713 using the location transforminformation 711.

FIG. 22 is a flow chart that may be used in using the basic Bowers orEaton method. The input data 801comprise logs (e.g., sonic, density,gamma ray, porosity), pressure measurements in a well (RFT, MDT, mudweight, LOT), velocity functions, seismic horizons, elevations (for landdata) and bathymetry (for marine data) and seismic data. Shale units areselected 803 as described above with reference to FIG. 2.Density-velocity and porosity-velocity relationships may be derivedinteractively 805. Overburden stresses are obtained from log data 807and calibrations using crossplots are derived from the velocity,pressure and overburden stress 809: the Eaton trend line of the Bowersmethod without unloading may be used. The derived calibrations are usedto predict the pore pressure, density, and/or porosity or other selectedparameters 811. The predictions of parameters of interest may beexported 819 or displayed on a seismic section 817. Basemaps may beproduced 815 using location transforms 813.

The flow chart of FIG. 23 shows input data 901 comprising logs (e.g.,sonic, density, gamma ray, porosity), pressure measurements in a well(RFT, MDT, mud weight, LOT), velocity functions, seismic horizons,elevations (for land data) and bathymetry (for marine data) and seismicdata. Plots of logs are obtained, selected by lithology or by boundaries903. Density-velocity and porosity-velocity trends may be obtained 905.An overburden calculation is made 907 using log data. Seismic velocitiesare obtained between defined horizons 908 and when combined with theoverburden data 907 can be used to derive calibrations incorporatingboundary constraints as well as the effects of unloading 909. Thecalibrations 909 are then used to predict attributes 911 that may thenbe exported 919 or displayed on a seismic section 917. As in FIGS.22-23, location transforms may be applied 913 and basemaps produced.

The foregoing description has been limited to specific embodiments ofthis invention. It will be apparent, however, that variations andmodifications may be made to the disclosed embodiments, with theattainment of some or all of the advantages of the invention. Therefore,it is the object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A method for determining a parameter of interestof a subsurface region of earth formations comprising: (a) obtainingseismic survey information about the subsurface region; (b) identifyinga plurality of interpreted seismic horizons of interest from theobtained survey information; (c) obtaining estimated seismic velocitiescorresponding to at least one interval between at least one pair of saidplurality of seismic horizons; (d) calibrating the estimated seismicvelocities to the parameter of interest (e) using the results of saidcalibration and the obtained seismic velocities to obtain the parameterof interest at any location within the seismic survey.
 2. The method ofclaim 1 wherein calibrating the estimated seismic velocities furthercomprises at least one of: (i) using a function determined independentlyfrom the seismic data using regional information; (ii) using data from awell corresponding to a specific calibration location that is outsidethe areal extent of the seismic velocity data; (iii) using data from awell corresponding to a specific calibration location within the arealextent of the seismic velocity data; (iv) using data from a wellcorresponding to a specific calibration location within the areal extentof the seismic velocity data combined with the velocity data from theseismic survey for the same said location.
 3. The method of claim 2where the calibrating the estimated seismic velocities includes theestimation of an overburden-depth relationship that is determined byintegrating density data from at least one of: I. a 1-D density functionderived from density logs from at least one well; II. a 1-D densityfunction derived from density data from drop cores, side-wall cores orconventional cores; III. a spatially varying density function based onthe inversion of potential fields data, said potential fields datacomprising at least one of gravity data, and magnetic data; IV. adensity function derived by performing inversion of at least one of PPdata, and PS data.
 4. The method of claim 1 wherein obtaining estimatedseismic velocities further comprises using specific seismicpre-processing techniques including at least one of: (i) Dip Move out(DMO) (ii) Pre-stack time migration (iii) Pre-stack depth migration (iv)Multiple attenuation or suppression (v) Reflectionstatics (vi)Refraction statics (vii) Wavefield reconstruction (viii) The combinationin undersampled seismic data from several gathers to form super-gathers.5. The method of claim 1 wherein the parameter of interest is selectedfrom the group consisting of: (i) fluid pressure (ii) fracture gradient(iii) overburden pressure (iv) effective stress (v) excess pressure,defined as the difference between the actual fluid pressure and thenormal pressure for the same depth.
 6. The method of claim 5 furthercomprising using the parameter of interest for performing furtheranalysis at one of the following scales: I. the scale of a specific welllocation, defined as a projected one-dimensional path that will be usedto place a wellbore into the subsurface II. the scale of an explorationprospect III. the scale of a regional pressure evaluation to understandhydrocarbon systems.
 7. The method of claim 5 further comprising usingthe parameter of interest for performing specific analyses of thesubsurface including at least one of the following: I. basin modelingII. hydrocarbon maturation in source rock intervals III. lateral andvertical seal rock integrity IV. fault seal integrity V. evaluation offluid migration pathways in the subsurface VI. reservoir-specificlateral pressure changes VII. shallow water flow risk evaluation VIII.wellbore stability and failure analysis.
 8. The method of claim 1wherein the seismic survey comprises one of: (i) P-wave land seismicdata (ii) S-wave land seismic data (iii) mode-converted S-wave land data(land PS data) (iv) mode-converted S-wave marine ocean-bottom cable data(marine PS data) (v) P-wave marine streamer data (vi) P-wave marineocean bottom cable data (PP data).
 9. The method of claim 1 wherein theobtained seismic velocities are selected from the group consisting of:(i) P-wave velocity data generated from normal moveout (NMO) velocityanalysis (ii) P-wave or S-wave velocity data generated from coherencyinversion analysis (iii) P-wave velocity generated from pre-stackinversion (iv) P-wave velocity generated from post-stack inversion (v)S-wave velocity generated from pre-stack inversion (vi) S-wave velocitygenerated from post-stack inversion (vii) S-wave velocity data generatedfrom normal moveout (NMO) velocity analysis (viii) P-wave velocitygenerated from tomography (ix) S-wave velocity generated from tomography(x) P-wave velocity data from vertical seismic profiling (VSP) (xi)P-wave velocity data from VSP look-ahead inversion (xiii) S-wavevelocity data from vertical seismic profiling (VSP) (xiv) S-wavevelocity data from VSP look-ahead inversion.
 10. The method of claim 2wherein the data from a well comprises at least one of: I. sonic logsII. shear sonic logs III. density logs IV. Lithology logs in the form ofgamma ray or Spontaneous Potential logs V. formation fluid pressure data(further dependent claims) VI. formation fracture pressures in the formof Leak Off Tests.
 11. The method of claim 1 wherein using the resultsof said calibration and the obtained seismic velocities to obtain theparameter of interest further comprises using at least one of: (ii) theEaton method (ii) the effective stress method (iii) the equivalent depthmethod.
 12. The method of claim 1 wherein calibrating the estimatedseismic velocities further comprises using at least one of: (i) a linearcurve fitting (ii) a curve fitting based on a power law (iii) a curvefitting based on exponentials, (iv) a quadratic curve fitting.
 13. Themethod of claim 12 wherein calibrating the estimated seismic velocitiesfurther comprises: using an interactive display, said interactivedisplay allowing modification of a displayed calibration curve andobserving changes in a plurality of other displayed curvessimultaneously, said interactive display further allowing at least oneof: I. manually changing a coefficient or exponent in the fittingequation, and II. using a cursor to graphically manipulate the fittingcurve.
 14. The method of claim 1 further comprising: obtaining aporosity calibration curve from well data using at least one of: (i)editing of erroneous porosity values for at least one well; (ii)displaying in a single plot, a depth-correlated porosity and velocitydata for the at least one well; (iii) curve fitting of the velocity andporosity data using at least one of: I. a linear curve fitting II. acurve fitting based on a power law III. a curve fitting based onexponentials, IV. a quadratic curve fitting; and predicting porosity ata specified location using the obtained porosity calibration curve andthe seismic velocity data from the specified location.
 15. The method ofclaim 1 wherein the parameter of interest is a fracture gradient, themethod further comprising: (i) deleting erroneous leak-off test valuesfor at least one well; (ii) displaying depth-correlated leak-off testand overburden data for the at least one well, and (iii) applying one ofthe two following methods I. curve fitting of the leak off test data todetermine a fracture gradient, and II. applying an appropriate stressratio (Ko) to fit the leak-off test data.
 16. The method of claim 1further comprising displaying the parameter of interest on one of: (i)stacked seismic section, and (ii) migrated seismic section.
 17. Themethod of claim 1 further comprising displaying the parameter ofinterest interactively and simultaneously on at least one of: (i) aseismic display; (ii) a velocity versus depth display including avelocity function for a specific location and a calibration function forvelocity versus effective stress, (iii) a stress versus depth displayincluding the overburden stress calibration for said specific locationand the effective stress calculated from the velocity versus depthdisplay, (iv) a pressure-gradient versus depth display including thefracture gradient or overburden gradient, the fluid pressure gradientcalculated from the stress versus depth display, and pressure datapoints from a well that applies to said specific location.
 18. Themethod of claim 1 wherein the at least one pair of the plurality ofseismic horizons comprises at least two pairs, the method furthercomprising performing step (d) of claim 1 separately for each of the atleast two pairs of seismic horizons.
 19. The method of claim 1 furthercomprising displaying the parameter of interest in at least one of: (i)a tabular display; and (ii) a graphical display.
 20. The method of claim1 wherein calibrating the estimated seismic velocities furthercomprises: (i) deleting or correcting for zones of abnormal velocitycaused by the presence of hydrocarbon fluids; and (ii) deleting zones ofabnormal velocity caused by the presence of nonelastic rocks.
 21. Themethod of claim 1 further comprising at least one of: (i) determining anoverburden stress from a density function based on inversion of 2-D or3-D potential fields; (ii) determining an overburden stress from adensity function derived by performing a simultaneous inversion of PPand PS data; (iii) correcting for zones of abnormal density caused bythe presence of hydrocarbon fluids by inserting a correct density forsaid zones and recalculating the overburden at the specific location;(iv) correcting for zones of abnormal velocity caused by the presence ofhydrocarbon fluids by deleting said zones from the calculation of theparameter of interest; (v) correcting for zones of abnormal densitycaused by the presence of non-clastic rocks by inserting a correctdensity for said zones and recalculating the overburden at the specificlocation; and (vi) correcting for zones of abnormal velocity caused bythe presence of non-clastic rocks by deleting said zones from thecalculation of the parameter of interest.
 22. The method of claim 1further comprising at least one of: (i) using an interactive help menu;(ii) performing a coordinate transformation; and (iii) automaticallypropagating results of an editing.
 23. The method of claim 3 wherein thecalibration includes the estimation of an overburden-depth relationshipthat is determined by integrating density data obtained by inversion of2-D or 3-D potential fields data.
 24. The method of claim 3 wherein thecalibration includes the estimation of an overburden-depth relationshipthat is determined by integrating density data obtained by inversion ofat least one of 2-D or 3-D seismic data, and wherein said seismic datafurther comprise at least one of PP data and PS data.
 25. A method fordetermining a parameter of interest of a subsurface region of earthformations comprising: (a) obtaining seismic survey information aboutthe subsurface region; (b) obtaining estimated seismic velocitiescorresponding to at least one interval of the subsurface from: (i)coherency inversion analysis of P-wave or S-wave seismic data; (ii)pre-stack inversion of P-wave seismic data; (iii) post-stack inversionof P-wave seismic data; (iv) pre-stack inversion of S-wave seismic data;(v) post-stack inversion of S-wave seismic data; (vi) normal moveout(NMO) velocity analysis of S-wave seismic data; (vii) tomographicanalysis of P-wave seismic data; (viii) tomographic analysis of S-waveseismic data; (ix) analysis of P-wave data from vertical seismicprofiling (VSP) data; (x) analysis of P-wave data from inversion of VSPlook-ahead data; (xi) analysis of S-wave data from vertical seismicprofiling (VSP) data (xii) analysis of S-wave data from inversion of VSPlook-ahead data; (c) calibrating the estimated seismic velocities to theparameter of interest (d) using the results of said calibration and theobtained seismic velocities to obtain the parameter of interest at anylocation within the seismic survey.
 26. The method of claim 25 whereincalibrating the estimated seismic velocities further comprises at leastone of: (i) using a function determined independently from the seismicdata using regional information; (ii) using data from a wellcorresponding to a specific calibration location that is outside theareal extent of the seismic velocity data; (iii) using data from a wellcorresponding to a specific calibration location within the areal extentof the seismic velocity data; (iv) using data from a well correspondingto a specific calibration location within the areal extent of theseismic velocity data combined with the velocity data from the seismicsurvey for the same said location.
 27. The method of claim 26 wherecalibrating the estimated seismic velocities includes further comprisesestimating an overburden-depth relationship by integrating density datafrom at least one of: I. a 1-D density function derived from densitylogs from at least one well; II. a 1-D density function derived fromdensity data from drop cores, side-wall cores or conventional cores;III. a spatially varying density function based on the inversion ofpotential fields data, said potential fields data comprising at leastone of gravity data, and magnetic data; IV. a density function derivedby performing an inversion of at least one of PP data, and PS data. 28.The method of claim 25 wherein obtaining seismic velocity data furthercomprises using seismic processing techniques that include at least oneof: (i) Dip Move out (DMO) (ii) Pre-stack time migration (iii) Pre-stackdepth migration (iv) Multiple attenuation or suppression (v) Reflectionstatics (vi) Refraction statics (vii) Wavefield reconstruction (viii)The combination in undersampled seismic data from several gathers toform super-gathers.
 29. The method of claim 25 wherein the parameter ofinterest is selected from the group consisting of: (i) fluid pressure(ii) fracture gradient (iii) overburden pressure (iv) effective stress(v) excess pressure, defined as the difference between the actual fluidpressure and the normal pressure for the same depth.
 30. The method ofclaim 25 further comprising using the parameter of interest to performfurther analysis at one of the following scales: I. the scale of aspecific well location, defined as a projected one-dimensional path thatwill be used to place a wellbore into the subsurface II. the scale of anexploration prospect III. the scale of a regional pressure evaluation tounderstand hydrocarbon systems.
 31. The method of claim 26 furthercomprising using the parameter of interest for performing specificanalyses of the subsurface including at least one of the following: I.basin modeling II. hydrocarbon maturation in source rock intervals III.lateral and vertical seal rock integrity IV. fault seal integrity V.evaluation of fluid migration pathways in the subsurface VI.reservoir-specific lateral pressure changes VII. shallow water flow riskevaluation VIII. wellbore stability and failure analysis.
 32. The methodof claim 25 wherein the seismic survey comprises one of: (i) P-wave landseismic data (ii) S-wave land seismic data (iii) mode-converted S-waveland data (land PS data) (iv) mode-converted S-wave marine ocean-bottomcable data (marine PS data) (v) P-wave marine streamer data (vi) P-wavemarine ocean bottom cable data (PP data).
 33. The method of claim 26wherein the data from a well comprises at least one of: I. sonic logsII. shear sonic logs III. density logs IV. Lithology logs in the form ofgamma ray or Spontaneous Potential logs V. formation fluid pressure data(further dependent claims) VI. formation fracture pressures in the formof Leak Off Tests.
 34. The method of claim 25 wherein using the resultsof said calibration and the obtained seismic velocities to obtain theparameter of interest further comprises using at least one of: (ii) theEaton method (ii) the effective stress method (iii) the equivalent depthmethod.
 35. The method of claim 25 wherein calibrating the estimatedseismic velocities further comprises using at least one of: (i) a linearcurve fitting (ii) a curve fitting based on a power law (iii) a curvefitting based on exponentials (iv) a quadratic curve fitting.
 36. Themethod of claim 26 wherein calibrating the estimated seismic velocitiesfurther comprises: using an interactive display, said interactivedisplay allowing modification of a displayed calibration curve andobserving changes in a plurality of other displayed curvessimultaneously, said interactive display further allowing at least oneof: I. manually changing a coefficient or exponent in the fittingequation, and II. using a cursor to graphically manipulate the fittingcurve.
 37. The method of claim 25 further comprising: obtaining aporosity calibration curve from well data using at least one of: (i)editing of erroneous porosity values for at least one well; (ii)displaying in a single plot, a depth-correlated porosity and velocitydata for the at least one well; (iii) curve fitting of the velocity andporosity data using at least one of: I. a linear curve fitting II. acurve fitting based on a power law III. a curve fitting based onexponentials, IV. a quadratic curve fitting; and predicting porosity ata specified location using the obtained porosity calibration curve andthe seismic velocity data from the specified location.
 38. The method ofclaim 25 wherein the parameter of interest is a fracture gradient, themethod further comprising: (i) deleting erroneous leak-off test valuesfor at least one well; (ii) displaying depth-correlated leak-off testand overburden data for the at least one well, and (iii) applying one ofthe two following methods: I. curve fitting of the leak off test data todetermine a fracture gradient, and II. applying an appropriate stressratio (Ko) to fit the leak-off test data.
 39. The method of claim 25further comprising displaying the parameter of interest on one of: (i)stacked seismic section, and (ii) migrated seismic section.
 40. Themethod of claim 25 further comprising displaying the parameter ofinterest interactively and simultaneously on at least one of: (i) aseismic display (ii) a velocity versus depth display including avelocity function for a specific location and a calibration function forvelocity versus effective stress, (iii) a stress versus depth displayincluding the overburden stress calibration for said specific locationand the effective stress calculated from the velocity versus depthdisplay, (iv) a pressure-gradient versus depth display including thefracture gradient or overburden gradient, the fluid pressure gradientcalculated from the stress versus depth display, and pressure datapoints from a well that applies to said specific location.
 41. Themethod of claim 25 wherein the at least one pair of the plurality ofseismic horizons comprises at least two pairs, the method furthercomprising performing step (d) of claim 1 separately for each of the atleast two pairs of seismic horizons.
 42. The method of claim 25 furthercomprising displaying the parameter of interest in at least one of: (i)a tabular display; and (ii) a graphical display.
 43. The method of claim25 wherein calibrating the estimated seismic velocities at a specificlocation further comprises: (i) deleting or correcting for zones ofabnormal velocity caused by the presence of hydrocarbon fluids; and (ii)deleting zones of abnormal velocity caused by the presence of nonelasticrocks.
 44. The method of claim 25 further comprises using at least oneof: (i) determining an overburden stress from a density function basedon inversion of 2-D or 3-D potential fields; (ii) determining anoverburden stress from a density function derived by performing asimultaneous inversion of PP and PS data; (iii) correcting for zones ofabnormal density caused by the presence of hydrocarbon fluids byinserting a correct density for said zones and recalculating theoverburden at the specific location; (iv) correcting for zones ofabnormal velocity caused by the presence of hydrocarbon fluids bydeleting said zones from the calculation of the parameter of interest;(v) correcting for zones of abnormal density caused by the presence ofnon-clastic rocks by inserting a correct density for said zones andrecalculating the overburden at the specific location; and (vi)correcting for zones of abnormal velocity caused by the presence ofnon-clastic rocks by deleting said zones from the calculation of theparameter of interest.
 45. The method of claim 25 further comprising atleast one of: (i) using an interactive help menu; (ii) performing acoordinate transformation; and (iii) automatically propagating resultsof an editing.
 46. The method of claim 27 wherein the calibrationincludes the estimation of an overburden-depth relationship that isdetermined by integrating density data obtained by inversion of 2-D or3-D potential fields data.
 47. The method of claim 27 wherein thecalibration includes the estimation of an overburden-depth relationshipthat is determined by integrating density data obtained by inversion ofat least one of 2-D or 3-D seismic data, and wherein said seismic datafurther comprise at least one of PP data and PS data.
 48. A method fordetermining fluid pressure in a subsurface region of earth formationscomprising: (a) obtaining seismic survey information about thesubsurface region; (b) identifying a plurality of interpreted seismichorizons of interest from the obtained survey information; (c) obtainingestimated seismic velocities corresponding to at least one intervalbetween at least one pair of said plurality of seismic horizons; (d)calibrating the estimated seismic velocities to the parameter ofinterest (e) using the results of said calibration and the obtainedseismic velocities to obtain the parameter of interest at any locationwithin the seismic survey.
 49. A method for determining an effectivestress in a subsurface region of earth formations comprising: (a)obtaining seismic survey information about the subsurface region; (b)identifying a plurality of interpreted seismic horizons of interest fromthe obtained survey information; (c) obtaining estimated seismicvelocities corresponding to at least one interval between at least onepair of said plurality of seismic horizons; (d) calibrating theestimated seismic velocities to the parameter of interest (e) using theresults of said calibration and the obtained seismic velocities toobtain the parameter of interest at any location within the seismicsurvey.
 50. A method for determining fluid pressure in a subsurfaceregion of earth formations comprising: (a) obtaining seismic surveyinformation about the subsurface region; (b) obtaining estimated seismicvelocities corresponding to at least one interval of the subsurfacefrom: (i) coherency inversion analysis of P-wave or S-wave seismic data;(ii) pre-stack inversion of P-wave seismic data; (iii) post-stackinversion of P-wave seismic data; (iv) pre-stack inversion of S-waveseismic data; (v) post-stack inversion of S-wave seismic data; (vi)normal moveout (NMO) velocity analysis of S-wave seismic data; (vii)tomographic analysis of P-wave seismic data; (viii) tomographic analysisof S-wave seismic data; (ix) analysis of P-wave data from verticalseismic profiling (VSP) data; (x) analysis of P-wave data from inversionof VSP look-ahead data; (xi) analysis of S-wave data from verticalseismic profiling (VSP) data (xii) analysis of S-wave data frominversion of VSP look-ahead data; (c) calibrating the estimated seismicvelocities to the parameter of interest (d) using the results of saidcalibration and the obtained seismic velocities to obtain the parameterof interest at any location within the seismic survey.
 51. A method fordetermining an effective stress in a subsurface region of earthformations comprising: (a) obtaining seismic survey information aboutthe subsurface region; (b) obtaining estimated seismic velocitiescorresponding to at least one interval of the subsurface from: (i)coherency inversion analysis of P-wave or S-wave seismic data; (ii)pre-stack inversion of P-wave seismic data; (iii) post-stack inversionof P-wave seismic data; (iv) pre-stack inversion of S-wave seismic data;(v) post-stack inversion of S-wave seismic data; (vi) normal moveout(NMO) velocity analysis of S-wave seismic data; (vii) tomographicanalysis of P-wave seismic data; (viii) tomographic analysis of S-waveseismic data; (ix) analysis of P-wave data from vertical seismicprofiling (VSP) data; (x) analysis of P-wave data from inversion of VSPlook-ahead data; (xi) analysis of S-wave data from vertical seismicprofiling (VSP) data (xii) analysis of S-wave data from inversion of VSPlook-ahead data; (c) calibrating the estimated seismic velocities to theparameter of interest (d) using the results of said calibration and theobtained seismic velocities to obtain the parameter of interest at anylocation within the seismic survey.